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Method for focus adjustment in a ct apparatusRelated Patent Categories: Radiant Energy, Radiation Controlling MeansMethod for focus adjustment in a ct apparatus description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070023713, Method for focus adjustment in a ct apparatus. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention concerns a method for focus adjustment in a CT (computed tomography) apparatus of the type having at least one x-ray tube with a springing focus, that x-ray tube rotating around a system axis, and a multi-line detector with a plurality of detector elements, is situated opposite the x-ray tube. [0003] 2. Description of the Prior Art [0004] CT apparatus of the above type is known wherein the focus on the anode changes position in one dimension or in two dimensions with a specific focus jump frequency. The output signals of the detector elements of the detector are integrated with a specific detector sampling frequency. The focus jump frequency is equal to the detector sampling frequency; and the jumped-to focus positions and the phase shift are set between the focus jump frequency and the detector sampling frequency. [0005] It is generally known that a springing focus arrangement can be used to improve the spatial resolution of a CT apparatus. Using a magnetic deflection system in the x-ray tube, the radiation-generating electron beam is influenced such that a plurality of positions on the anode can be jumped to with a specific springing focus frequency, and thereby each detector element can be irradiated from different directions and exposed multiple times in a projection. The sampling of the detector signals ensues with a frequency that results from the plurality of the projections per rotation of the gantry, the rotation speed and the plurality of the different springing focus positions. If the sampling ensues with suitably shifted focal point, the effective sampling rate can be multiplied and the resolution and quality of the CT examination can be improved. [0006] The focus deflection can be implemented both in the axial direction (Z-direction=system axis direction) and in the transaxial direction (.phi.-direction=circumferential direction=azimuthal direction) or in combinations of both directions. In each case it is necessary to determine the phase relation between the deflection movement of the focus (focus springing frequency) and the frequency of the data acquisition (detector sampling frequency), the deflection difference in the springing focus positions and the absolute focus position on the anode (focus offset). [0007] For example, by correct selection of the deflection of the springing focus (focus offset) in the azimuthal direction two positions are achieved (known as focus alignment in the rotation center of the gantry) as well as a quadruple sampling of the detector pixels in the rotation center in connection with an azimuthal springing focus and a known 1/4 offset of the detector elements. A transfer frequency of the imaging system (focus/detector system) that is four times higher is thus enabled. [0008] For this purpose it is necessary to carefully match the alignment of the springing focus positions and the phase shift between the focus springing frequency and the detector sampling frequency. In principle, these quantities can be determined from CT scans with orbital sampling and a suitable phantom in a springing focus operation. An eccentrically positioned, strongly attenuating small sphere or (if the springing focus jumps only in the azimuthal direction) a small cylinder is suitable as a phantom. This matching is conventionally achieved by iterative approximation of the optimal deflection of the focus and the optimal phase with multiple measurements and intervening evaluations. [0009] Such iterative methods are very time-consuming and costly and there is need to simplify and shorten such procedures. SUMMARY OF THE INVENTION [0010] An object of the present invention is to provide a method for focus adjustment in a CT apparatus with a springing focus that operates sufficiently well with a minimum of measurement and time expenditure. [0011] The invention is based on the recognition that, with the acquisition of a low plurality of sinograms of a phantom with different parameters with regard to the position of the focus, and the phase between the detector sampling frequency and the focus springing frequency, it is possible to determine the correct correction values for the jumped-to focus positions and the phase adjustment directly, and without an iterative procedure. [0012] Based on this recognition, the above object is achieved by a method for non-iterative focus adjustment in a CT apparatus that has at least one x-ray tube with a springing focus, the x-ray tube rotating around a system axis, and a multi-line detector with a plurality of detector elements that is situated opposite the at least one x-ray tube, wherein the focus changes position on the anode in one dimension or in two dimensions with a specific focus springing frequency; the output signals of the detector elements of the detector being integrated with a specific detector sampling frequency, and the focus springing frequency being equal to the detector sampling frequency, and the position of the center ray being calculated with regard to the movement direction of the focus and the phase between the detector sampling frequency, and the focus springing frequency is determined and adjusted by predetermined steps. At least three sinograms of a spherical or cylindrical absorber are acquired at positions of the focus in one dimension that alternate during the acquisition, or at least five sinograms of a spherical or cylindrical absorber are acquired at positions of the focus in two dimensions that alternate during the acquisition. The sinograms are acquired with different parameters with regard to the position of the focus and the phase between the detector sampling frequency and the focus springing frequency, and furthermore from the acquired sinograms. [0013] Iterative steps thus are not needed with and the correct or optimal phase can be set. The specified minimum values of measurement sinograms represent the mathematically necessary minimum values for an exact calculation, in which linear relationships must exist in the range of the measurement values. Although the invention is implemented without an iterative procedure when a larger plurality of measurements is conducted an improvement in the precision is achieved. This means that all measurements are conducted with the same conditions, thus the same focus offset, and no adjustment of the focus offset and no approximation to a selected phase ensues between the measurements. [0014] The measurement results to be supplemented by the focus deflections that are necessary for determination of the desired focus positions and phase, being determined by scaling with the focus deflection and interpolation. [0015] The following formula can be used for scaling the focus deflection in one dimension: ( .THETA. 1 - .THETA. 2 ) .times. ( .tau. i , .rho. k ) = ( .THETA. 1 - .THETA. 2 ) .times. ( .tau. i , .rho. i ) .rho. k .rho. i , with .times. .times. 1 .ltoreq. k , i .ltoreq. N wherein .THETA..sub.1 and .THETA..sub.2 are the alignments of the first and second focus positions, .tau..sub.i is the phase between the detector sampling frequency f.sub.D and the focus springing frequency f.sub.F, .rho..sub.k is a value proportional to the focus deflection (such as a deflection current), and the alignments .THETA..sub.1 and .THETA..sub.2 are calculated from the sinograms of the respective focus positions according to .intg..sub.0.sup.2.pi..beta.(.alpha.)d.alpha.=2.pi..THETA., wherein .beta.(.alpha.) corresponds to the emphasis line of the absorption of the absorber in the sinogram and .alpha. corresponds to the projection angle. [0016] An equidistant oversampling of the detector channels can be achieved by the interaction between a set detector offset and the set focus positions. [0017] Furthermore, the focus positions and the phase between the detector sampling frequency and the focus springing frequency can be determined by interpolation using a trapezoidal time function. [0018] The invention is non-iterative for determination of phase, amplitude and offsets of the dynamic focus deflection. Thus the focus alignment adjustment can be significantly accelerated. By suitable establishment of different values for the phase, deflection amplitude and focus offset, the optimal values can be calculated from sequential measurements with a suitable phantom. [0019] For explanation of the invention, the scanning of a subject with a CT apparatus with springing focus is described. Initially only a springing focus with two different focus positions in the azimuthal direction is described, and subsequently an expansion of the springing focus in the system axis direction is described. DESCRIPTION OF THE DRAWINGS [0020] FIG. 1 is an overview of a CT apparatus. [0021] FIG. 2 schematically shows a normal focus of a CT apparatus with the detector situated opposite thereto in the vertical position with 1/8 offset. Continue reading about Method for focus adjustment in a ct apparatus... Full patent description for Method for focus adjustment in a ct apparatus Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method for focus adjustment in a ct apparatus patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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